<?php $numbers = array_map('intval', explode("\n", file_get_contents('input.txt'))); $current_index = 0; $step = 25; while (true) { for ($l = count($numbers) - $step; $current_index <= $l; $current_index++) { $number_to_test = $numbers[$current_index + $step]; $numbers_to_addition = array_slice($numbers, $current_index, $step); foreach ($numbers_to_addition as $number) { if (in_array($number_to_test - $number, $numbers_to_addition, true)) { $current_index++; break 2; } } echo $number_to_test; exit; } }